This chapter is from the book

Planning an Internet Connection

Consider your company's needs when planning your Internet connection. If all you need is one-way connectivity to enable internal users to connect to sites on the Internet, a private IP address space with Network Address Translation (NAT) should suffice. If external users need to connect to resources, such as servers, inside your network, you need some public IP addresses. You might combine these with private addresses and NAT for your users.

If external users must connect to your internal resources, you should plan the following:

How many public IP addresses will you need?

Should you get your IP addresses from your ISP or acquire your own? If you elect to acquire your own addresses, you also need a public Autonomous System (AS) number.

What link type and speed will you need to support all the external connections plus your internal users?

Will you use static or dynamic routing?

How much redundancy will you need? This includes link redundancy and ISP redundancy.

To Route or Not to Route?

If your ISP connection is a Layer 2 circuit emulation, there is no need to run a routing protocol with the ISP.

If you use MPLS VPNs, you either use static routes or run a dynamic routing protocol with the ISP edge router. This might be either one of the IGPs (EIGRP, OSPF, or RIP) or Border Gateway Protocol (BGP).

If you need only a default route pointing to your ISP, static routes work. The provider needs to create static routes pointing to your network and redistribute them into its routing protocol.

BGP is a good choice if you connect to multiple ISPs, you need to control how traffic enters or exits your company, or you need to react to Internet topology changes.

BGP Route Options

You have a choice of three ways to receive BGP routes from an ISP:

Default routes from each provider: This is simple to configure and results in low use of bandwidth and router resources. The internal network's IGP metric determines the exit router for all traffic bound outside the autonomous system. No BGP path manipulation is possible, so this can lead to suboptimal routing if you use more than one ISP.

Default routes plus some more specific routes: This option results in medium use of bandwidth and router resources. It enables you to manipulate the exit path for specific routes using BGP so that traffic takes a shorter path to networks in each ISP. Thus path selection is more predictable. The IGP metric chooses the exit path for default routes.

All routes from all providers: This requires the highest use of bandwidth and router resources. It is typically done by large enterprises and ISPs. Path selection for all external routes can be controlled via BGP policy routing tools.

Types of ISP Connections

A site with a single ISP connection is single-homed. This is fine for a site that does not depend heavily on Internet or WAN connectivity. Either use static routes, or advertise the site routes to the ISP and receive a default route from the ISP.

A dual-homed site has two connections to the same ISP, either from one router or two routers. One link might be primary and the other backup, or the site might load balance over both links. Either static or dynamic routing would work in this case.

Multihoming means connecting to more than one ISP at the same time. It is done for redundancy and backup if one ISP fails, and for better performance if one ISP provides a better path to frequently used networks. This also gives you an ISP-independent solution. BGP is typically used with multihomed connections.

You can take multihoming a step further and be dual-multihomed, with two connections to multiple ISPs. This gives the most redundancy. BGP is used with the ISPs and can be used internally also.